Can you check that in your custom code you have not overlayed any OOTB
libs files ?Similar issue was faced where below 2 files were overlayed,
post merging both (latest libs file + custom code), parsys issue got
resolved./libs/wcm/core/components/init/init.jsp/libs/foundation/global.jspIt
should not be replicable in fresh AEM 6.4 instance, when we dont have
any overlayed code.